On the call, which was previewed to NBC News this morning, Trump slams Rick Santorum as someone who "doesn't know about creating jobs," and is "completely entrenched in the Washington culture."

"I'm tired of Rick Santorum pretending he's some kind of DC outsider," Trump says at the start of the recording.

Trump spokesman Michael Cohen says the call will go out statewide, beginning today, and will go on for several days.

Trump has been active on Romney's behalf over the last week -- taking part in a New York City fundraising effort and doing radio interviews in Michigan and Ohio in which he has praised the former Massachusetts governor and attacked his rivals, but not ruled out his own third party run should Romney fail to win the nomination.